The VITamin D and OmegA-3 TriaL (VITAL; NCT 01169259) is a randomized clinical trial in 25,874 U.S. men and women investigating whether taking daily dietary supplements of vitamin D3 (2000 IU) or fish oil (1 gram of omega-3 fatty acids) reduces the risk of developing cancer, heart disease, and stroke in people who do not have a prior history of these illnesses. This ancillary study is being conducted among a sub-cohort of 771 participants in VITAL and will test the efficacy and safety of high-dose vitamin D supplementation vs. placebo on skeletal health and body composition.